1. Field of the Invention
This invention relates to an apparatus and a method for preparing a thin film containing a material having a crystal structure and exhibiting an absorption in ultraviolet region, in particular a carbon thin film containing graphite as principal ingredient. More particularly, it relates to an apparatus for preparing carbon thin film that is provided with means for analyzing the state of carbon thin film incorporated into a micro-area of 1 xcexcm and less square and a method of preparing a carbon thin film comprised of feeding back the result of the analysis of the state of carbon and reflecting it to the carbon thin film preparing conditions.
2. Related Background Art
Graphite is an allotrope of carbon, having structure consisting of hexagonal reticular planes with the sp2 hybridized orbital and showing the following specific physical properties. While it shows a semimetallic electroconductivity, a thermal conductivity three times as high as that of copper, a very high elasticity and also a very high mechanical strength within the reticular planes of carbon, its electric and thermal conductivities fall remarkably across the layers of its multilayer structure. Thus, it is exceptionally anisotropic if viewed along the reticular planes and a direction perpendicular to the planes.
A variety of different models have been proposed for the layered structure of reticular planes of carbon. For example, Franklin defines a turbostratic structure where reticular planes are randomly laid one on the other and a graphite crystal structure where reticular planes are arranged in a coordinated manner and says that each distance separating the reticular planes is 0.344 nm for the turbostratic structure and 0.335 nm for the graphite crystal (R. E. Franklin, Proc. Roy. Soc., A209, 196(1951)).
Because of the remarkable physical properties of graphite due to its particular structural feature including a high thermal resistance and a high chemical resistance, it finds a wide variety of industrial applications including refractory materials, materials of atomic furnaces and heat-emitting bodies. Additionally, highly-crystallized graphite shows excellent spectral and reflective characteristics for X-rays and neutron rays and hence are advantageously used for monochromators and filters.
Recently, a graphite intercalation compound obtained by utilizing interlayer spaces of graphite has been attracting attention and lithium ion cells prepared by utilizing a graphite intercalation compound are currently popular as small and high-performance secondary cells that find various practical applications. Other applications of the graphite include materials to be used for electronic circuits such as resistor coating film, adsorptive materials utilizing the porous structure of the graphite and electron emitting materials. Particularly, materials listed above and produced in recent years are of high quality, finely processed and thin. In response, there is a demand for techniques that can effectively analyze the structure, more specifically the average crystallite size, of the graphite used in a very fine area on the surface of a product comprising such a material.
The structure of graphite is typically defined in terms of the size of crystallites (crystallite size), using either the crystallite size Lc as observed along a direction perpendicular to the hexagonal reticular plane or the crystallite size La as observed along a direction parallel to the hexagonal reticular plane.
For the purpose of the present invention, the term xe2x80x9ccrystallitexe2x80x9d refers to a unitary crystal (microcrystal) that is a constituting member of polycrystal or a unitary crystal (microcrystal) that is observed in a noncrystalline substance. As far as this specification is concerned, the crystallite size of graphite refers to Lc as observed along a direction perpendicular to the hexagonal reticular plane.
It is known that graphite materials having different average crystallite sizes show physical properties that are remarkably different from each other, including the specific resistance, the thermal conductivity and the bending strength. For example, the specific resistance, the thermal conductivity and the bending strength will be respectively about 50xc3x9710xe2x88x924 xcexa9cm, about 3 kcal/mhrxc2x0 C. and about 900 kgf/cm2 for glassy carbon with an Lc of 10 nm, about 40xc3x9710xe2x88x924 xcexa9cm, about 7 kcal/mhrxc2x0 C. and about 1100 kgf/cm2 for glassy carbon with an Lc of 20 nm and about 10xc3x9710xe2x88x924 xcexa9cm, about 120 kcal/mhrxc2x0 C. and about 200 kgf/cm2 for artificial graphite. Clearly, graphite materials having different average crystallite sizes show physical properties that are remarkably different from each other. Therefore, in analyzing physical properties of a graphite material, it is indispensable to know the average crystallite size of the material. Conventionally, graphite, thin film containing graphite in particular, is produced by means of a thermal CVD, where a gaseous hydrocarbon compound is introduced onto a hot substrate to thermally decompose the gaseous compound and causes carbon in the decomposition product to precipitate in a vapor phase or a plasma CVD, where plasma is introduced into a reaction space to activate and decompose a gaseous hydrocarbon compound and causes carbon in the decomposition product to precipitate at relatively low temperature. Alternatively, graphite may be produced by heat treating a filmy polymeric compound. Known specific techniques for producing graphite include the one (as disclosed in Japanese Patent Publication No. 6-102531) with which a hydrocarbon compound such as methane is thermally decomposed by means of hot plasma to produce scale like film having a turbostratic crystal structure (the average crystallite size (Lc) is between that of graphite single crystal and that of amorphous carbon), the one (as disclosed in Japanese Patent Application Laid-Open No. 6-220638) with which a carbon coat is formed at relatively low temperature between 350 and 450xc2x0 C. by applying the catalytic function of nickel oxide to a hot CVD (so that nickel oxide may be carried on the surface of a substrate) and the one (as disclosed in Japanese Patent Application Laid-Open No. 5-17115) with which graphite film is prepared by laying a number of polymeric films that have been subjected to a preliminary oxidation treatment process into a multilayer structure, which is then pressed and heat-treated. Otherwise, there are also known techniques including the one (as disclosed in Japanese Patent Application Laid-Open No. 5-43213) with which a film of a polyimide compound having a fluorene molecular structure is pinched between a pair of graphite plates and baked to produce graphite and the one (as disclosed in Japanese Patent Application Laid-Open No. 5-78194) with which molten carbon on a metal column is made to precipitate on crystal seeds of graphite. There is also known a method of producing ultra-fine particles of graphite by vaporizing carbon through arc-discharge and subsequently cooling and solidifying the vaporized carbon (Japanese Patent Application Laid-Open No. 7-206416).
Known techniques for analyzing various graphite materials for determining the structure (crystal structure) include X-ray diffraction method, Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, high resolution electron energy loss spectroscopy, transmission-type electron energy loss spectroscopy and low energy electron diffraction method (see, for example, S. Aizawa, xe2x80x9cHyoumen Kagaku (Surface Science)xe2x80x9d, Vol. 11, No. 7, 398 (1990)).
X-ray diffraction method is a technique for determining the structure of a graphite material, using a diffraction pattern obtained by irradiating the specimen with X-rays and a computation model. With this technique, the distance between hexagonal reticular planes of the specimen is estimated on the basis of the diffraction peak, and the average crystallite size Lc as determined along a direction perpendicular to the hexagonal reticular planes of the specimen is estimated on the basis of the full width at half maximum B of the diffraction peak. The full width at half maximum B and the average crystallite size Lc show a relationship as defined by the formula below;
Lcxc2x7Bxc2x7cos xcex8=0.9xcex
where xcex8 is the Bragg angle and xcex is the wavelength of X-rays.
Raman spectroscopy is typically used for determining the structure of a graphite material on the basis of the spectrum of Raman scattering light from the specimen, using an ordinary laser as source of excitation. The average crystallite size La is estimated in a direction parallel to the hexagonal reticular planes from the center wave number and the full width at half maximum of the peak (1580 cmxe2x88x921, E2g mode) attributed to the graphite structure, those of the peak (at or around 1360 cmxe2x88x921, edge mode) reflecting the structural disorder of the specimen and their intensity ratio (see, for example, F. Tuinstra and J. L. Koenig, J. Chem. Phys., 53, 1126 (1970)).
With the analyzing technique using a transmission electron microscope, a thinned specimen of crystalline or granular graphite as thin as 100 nm is directly observed through a transmission electron microscope for analysis (and photographed on a silver salt film).
X-ray photoelectron spectroscopy is a technique of irradiating the specimen in ultra-vacuum with soft X-rays to analyze the kinetic energy of photoelectrons emitted from the surface of the specimen. This technique provides information on the chemical bond of carbon atoms to be obtained from the photoelectron peak energy, the X-ray excited Auger electron peak energy, and its profile, the existence or inexsistance of an energy loss peak, etc. The obtained information may include the existence or inexistence of Sp2 carbon from the energy-loss (inelastic) peak assigned to xcfx80xe2x86x92xcfx80* transitions and plasma, and the chemical state of Sp2 carbon, if it exists.
Ultraviolet photoelectron spectroscopy provides information mainly on phonons including the dispersion relations of graphite n-bands, in which the kinetic energy of photoelectrons, emitted from the surface of the specimen when irradiating the specimen with ultraviolet rays (approximately 10 eV) in ultra-vacuum, are analyzed.
High resolution electron energy loss spectroscopy is a technique for analyzing the energy of inelastic electrons when irradiating the specimen with monochromatic electron beams (approximately 10 eV). Basically, this technique provides information on phonons including the dispersion relations of graphite as the above described ultraviolet photoelectron spectroscopy.
Transmission-type electron energy loss spectroscopy is typically used for analyzing the state of a local area of a solid thin film specimen by observing the energy loss of electrons passing through the specimen by means of a transmission electron microscope. It has been reported by means of this technique that both the energy loss attributed to xcfx80xe2x86x92xcfx80* transitions and the energy loss attributed to plasmon observed in graphite vary remarkably depending on the crystallinity of graphite (L. B. Leder and J. A. Suddeth, J. Appl. Phys., 31, 1422 (1960)). More specifically, both the energy loss attributed to xcfx80xe2x86x92xcfx80* transitions and the energy loss attributed to plasmon increase with an increase of the crystallinity of graphite. It is also proved by computation in the above report that the above energy shifts are caused by differences in the density.
High resolution electron diffraction method is used to obtain information on the locational arrangement of atoms on the surface of a solid specimen by irradiating the specimen with electron beams having a wavelength substantially equal to the distance(s) separating atoms in the solid specimen (typically 100 eV or more) and observing the direction and the intensity of elastic electrons. If the specimen is graphite, the lattice constant and other characteristic parameters of the specimen can be estimated from ring-shaped pattern.
However, it is a known problem that graphite cannot be successfully formed in a micro-area of 1 xcexcm and less square in the process of producing a graphite thin film or a thin film containing graphite as an ingredient and incorporating it into an electronic device as part thereof, while controlling the structure of the graphite particularly in terms of crystallite size. Additionally, if a graphite thin film is successfully formed in a micro-area of 1 xcexcm and less square, the techniques that can be used for analyzing the crystallite size of the graphite are accompanied by the following problems.
Firstly, X-ray diffraction method, X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy adapted to utilize X-rays or ultraviolet rays that can hardly be converged cannot be used to analyze a micro-area of 1 xcexcm and less square. It is also difficult with the state of the art to converge a laser beam that is typically used for Raman spectroscopy to not more than 1 xcexcm. Thus, only electrons and ions are available for input radiation (probe) for analyzing the micro-area of 1 xcexcm and less square because they can be focused with ease.
Additionally, Raman spectroscopy that uses photons (normally of visible light) for detection signals is accompanied by a problem that it cannot be used to selectively analyze a surface layer with a depth of not more than 10 nm.
Now, the remaining known techniques that can be used for analyzing graphite formed in a micro-area will be briefly discussed for their shortcomings.
While the technique of transmission electron microscopy is adapted to analyze a micro-area in detail, it requires a long time for the preparation of a specimen (because the specimen should be as thin as about 10 nm). Therefore, if graphite to be observed contains crystallites with different sizes, a large number of particles will have to be observed to consume a long time.
Transmission-type electron energy loss spectroscopy is also accompanied by the problem of requiring a long time for the preparation of a specimen.
While the technique of high resolution electron energy loss spectroscopy is adapted to scrutinize the phonon dispersion of graphite on the surface, it is accompanied by the problem that no information can be obtained on the sizes of graphite crystallites with the level of energy normally used for input radiation. Additionally, it requires a specific monochromator for reducing primary electron beams monochromatic to make the analyzer inevitably a very bulky one.
Low energy electron diffraction method is adapted to obtain the lattice constant of graphite on the surface of a micro-area but it cannot directly acquire information on the crystallite sizes of graphite. Additionally, if the specimen is a thin graphite film, it can be affected by the underlay (multiple diffraction) to produce a complex diffraction pattern that cannot be analyzed without difficulty.
Analyzing techniques using a transmission electron microscope require a pretreatment of the specimen. If a specifically designed analyzer is used, the specimen has to be exposed to the atmosphere at the risk of contamination.
Thus, with any known process of producing a graphite thin film of a thin film containing graphite as an ingredient and incorporating it into an electronic device as part thereof, it is difficult to form graphite in a micro-area of 1 xcexcm and less square while controlling the structure of the graphite particularly in terms of crystallite size. It is also difficult to analyze the structure of the graphite thin film formed in a micro-area. Particularly, it is very difficult to determine the average particle size of the graphite formed in a micro-area of 1 xcexcm and less square in a surface layer with a depth of not more than 10 nm because there is no known technique of analyzing the structure of a graphite thin film formed in such a micro-area and it is difficult to effectively control the conditions under which such a thin film is formed in such a micro-area.
Therefore, it is the object of the present invention to provide a method and an apparatus for forming a thin film of graphite or other carbon material having a crystal structure in a micro-area of 1 xcexcm and less square under a controlled manner, while analyzing the structure of the thin film formed in the micro-area and determining the average particle size of the particles contained in the thin film.
According to an aspect of the present invention, there is provided a method of determining an average crystallite size of a material having a crystal structure and exhibiting an absorption in ultraviolet region, which comprises irradiating a specimen of the material with an electron beam and observing an energy loss spectrum of reflected electrons.
According to another aspect of the invention, there is provided an apparatus for preparing a thin film of a material having a crystal structure and exhibiting an absorption in ultraviolet region by means of a chemical vapor phase reaction, which comprises a container for preparing the thin film and a reflected electron energy loss spectroscopic analyzing system.
An apparatus for preparing a thin film of a material having a crystal structure according to the invention may also be characterized by comprising a system for controlling the conditions of preparing the thin film according to the signal from the reflected electron energy loss spectroscopic system.
According to still another aspect of the invention, there is provided a method of preparing a thin film of a material having a crystal structure and exhibiting an absorption in ultraviolet region by means of a chemical vapor phase reaction, which comprises analyzing a chemical state of the thin film by means of a reflected electron energy loss spectroscopic analyzing system without exposing it to the atmosphere, and improving a condition of preparing the thin film by feeding back an obtained result.
Preferably, in the analysis by means of reflected electron energy loss spectroscopy, the specimen is irradiated with electron beams and the energy loss spectrum of reflected electrons is observed to feed back the result of the analysis of the spectrum and reflect it to the conditions of preparing the thin film.